1. Field of the Invention
The present invention relates to electrical systems for vehicles, and more particularly, to an auxiliary battery system for providing back-up power to an electrical system of an aircraft.
2. Description of the Related Art
Vehicle electrical systems generally comprise a collection of electrical loads connected to a primary electrical bus, a rechargeable battery and source of power such as an engine driven alternator. FIG. 1A is a simplified block diagram of a prior art vehicle electrical system 10 having a primary electrical bus 12, a rechargeable battery 14, and an engine-drive alternator 15. The primary electrical bus 12 provides power to electrical loads 16 via a wire connection and interconnects the engine driven alternator 15 and rechargeable battery 14. The electrical loads 16 may comprise, for example, one or more lighting loads, a radio, a global positioning system (GPS) receiver, or an electronic flight information system (EFIS). The vehicle electrical system 10 may include a master contactor 18 that allows the primary electrical bus 12 to be disconnected from the rechargeable battery 14. When the vehicle electrical system 10 is de-energized, such that the engine driven alternator 15 is not producing power, the electrical loads 16 receive no power.
The prior art vehicle electrical system 10 shown in FIG. 1A has no true back-up source of power. The alternator 15 serves as the primary source of continuous power for the vehicle electrical system 10, while the rechargeable battery 14 ensures stability of the alternator 15 and serves as a temporary alternate source of power. During a failure of the alternator 15, the prior art vehicle electrical system 10 is limited to the endurance of the primary rechargeable battery 14. In the case of a failure of the master contactor 18, the prior art vehicle electrical system 10 may completely shut down because the alternator 15 may not be inherently stable without the primary rechargeable battery 14. Accordingly, the prior art vehicle electrical system 10 shown in FIG. 1A has limited use in aircraft electrical systems as it is prone to a single point of failure resulting in loss of electrical power.
FIG. 1B is a simplified block diagram of an enhanced prior art vehicle electrical system 20 including a second rechargeable battery 22 and a second contactor 24. The vehicle electrical system 20 of FIG. 1B provides the benefit of the redundancy of the second rechargeable battery 22 and the second contactor 24. However, as found in many aircraft, the second rechargeable battery 22 comprises a lead-acid (Pb-acid) type and is quite heavy and bulky, which are both highly undesirable qualities in airborne equipment.
The vehicle electrical system 20 of FIG. 1B is able to provide back-up power to all of the connected electrical loads 16 connected to the primary electrical bus 12. Unfortunately, during a failure of the alternator 15, all the electrical loads 16 will drain both of the rechargeable batteries 14, 22. Since many of the electrical loads 16 on the primary electrical bus 12 are often non-essential loads that can be safely shut-off during a failure of the alternator 15, it undesirable to have all of the electrical loads draining both of the rechargeable batteries 14, 22. In the vehicle electrical system 20 shown in FIG. 1B, it is up to the operator of the system to remember to shed (i.e., turn off) these loads to maximize operating duration of the batteries 14, 22.
FIG. 1C is a simplified block diagram of yet another prior art vehicle electrical system 30 that provides additional back-up power solutions. The vehicle electrical system 30 of FIG. 1C comprises an electrical load 32 that has an internal back-up battery 34, which provides complete independence from the primary electrical system (i.e., the alternator 15 and the batteries 14, 22). The vehicle electrical system 30 systematically ensures that each piece of equipment that requires back-up power has its own source of power. However, the vehicle electrical system 30 may require the maintenance of many back-up batteries 34, and additionally requires that the back-up batteries be located in very harsh environments, i.e., inside flight instruments, such as the EFIS system.
There is a need for a self-contained, light weight back-up battery system that provides power to critical electrical loads and operates automatically with respect to the condition of the primary electrical bus.